Railroad tracks are generally constructed on a base layer of compacted, crushed stone material. A layer of gravel ballast rests on top of this stone layer. Crossties are laid atop the base layer, and two parallel, flat-bottomed steel rails are attached to the crossties with fasteners, such as tieplates and spikes. After the rails are attached to the ties and the track has been checked for proper alignment, crushed stone ballast is then typically laid down between the ties to further support the ties and allow some adjustment of their position, while also allowing free drainage.
The majority of crossties in service are made of wood. Various other materials, such as concrete, steel, and composite or recycled material, are used in the manufacture of crossties. The crossties distribute the axle loads from the rails to the ballast layer below the crossties and contribute to the cushioning effect of the entire track structure. Over time, environmental factors can cause the crossties to deteriorate until they must be replaced. Annually, railroads in North America replace up to 2% or more of all wooden crossties. This constitutes several million crossties.
To manage the logistics of crosstie replacement and to quantify the need for new crossties, railroad inspectors attempt to grade the condition of crossties and the fastener system on a regular basis. This grading is most often done with a visual inspection to identify crossties and fasteners that are rotten, broken, split, or worn to an extent that their serviceable life is at or near its end. Additionally, the ballast on both sides of the railroad track must be inspected to ensure that it is properly graded, that there are no drainage problems, and/or other similar structural faults or problems. The process of visual inspection is quite time consuming. In practice, inspection of the track is performed by inspectors walking along the track to inspect and record the conditions of the crossties and/or fasteners, which are spaced approximately every 20 inches along the track. One particular North American railroad reports that a crew of three or four men can grade only about five to seven miles of track per day. With about 140,000 miles of railroad track in operational use in the United States today, that translates into a huge burden and cost for railroad maintenance. Therefore, systems of automatic and/or automated visual inspection, such as the Aurora vision system by Georgetown Rail Equipment Company of Georgetown, Tex., are extremely useful. Automatic inspection systems can be easily attached to a moving railroad car and can be configured to scan a railroad track while the cars are in motion. Such systems can use a scanning or profiling laser to determine a profile of railroad track components as the rail vehicle travels along the rails, with visual recognition systems that detect irregular profiles that indicate the presence of damaged or worn track components.
Optical inspection systems, like the Aurora vision system, can also be used for inspecting highways and streets. Road surfaces are principally made up of asphalt or concrete. Asphalt surfaces are constructed by combining a viscous bitumen binder with aggregate (e.g. sand, gravel and small stones). This surface is usually laid down upon a compacted, well-graded gravel base layer. Asphalt surfaces make up approximately 85 percent of the world's highway surfaces. On the other hand, concrete surfaces are usually a mixture of Portland cement, gravel, sand, and water. As is the case with asphalt road surfaces, concrete surfaces are often laid down on a compacted, well-graded gravel base layer. Concrete roadway surfaces can also include steel bars for reinforcement. Asphalt roadways have the advantage of being less expensive and permit significant plastic deformation before the surface fails. Concrete roadways, on the other hand, are significantly more expensive to install, but are typically stronger and more durable.
Both asphalt and concrete are susceptible to cracking, breaking, and structural failure caused in part by climatic effects and usage patterns. For instance, both asphalt and concrete are susceptible to deterioration due to freeze-thaw cycles. In addition to roadways made in asphalt and concrete, other relatively common surface materials include cobblestone and brick. These surfaces are also sensitive to environmental and use-based factors and prone to failure over time. For at least these reasons, it is important to regularly inspect and investigate roadway surfaces.
In the United States and China alone, there are over five million miles of paved highways. The task of manually performing a visual inspection of every mile of roadway is a tedious, time-consuming, and expensive task. Automatic inspection systems, like the Aurora system for railways, are an invaluable tool in the task of auditing the health of roads and highways and detecting cracks, flaws, and signs of failure.
However, it is desirable that the inspection system be properly calibrated in order to yield accurate results. Many vision calibration systems correct for common lens abnormalities such as linear distortion, barrel distortion, and pose estimation. However, since vision systems for scanning railroad tracks and other surfaces also need to recognize height variances, or spatial orientation, in the plane of the profiling laser in addition to across the surface and/or railroad tracks, it is desirable to calibrate such systems in both the laser and the surface or horizontal planes.
Image and vision calibration systems, often used in semiconductor lithography systems, for instance, usually only require calibration in one plane (referred to in this disclosure generally as the horizontal plane, though the plane is not necessarily completely horizontal), and therefore, a series of reference points, shapes, and other identifiable patterns can be used to calibrate in the horizontal plane. However, because railroad tracks and road surfaces have an additional depth component, representing the height of the metal rails, crossties, grooves, cracks, and joints in the surface of the roadway, and different varying depth components associated therewith, there is a need to calibrate in at least two planes.
It is also desirable to be able to calibrate a vision system outside of a laboratory or clean room environment, and without a fixed calibration target. Scanning a fixed target generally involves having a scanning system in motion, which can introduce errors into the calibration results. In addition, a fixed target can prove cumbersome and difficult to use outside of a laboratory environment.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.